The present invention relates to a high-dynamics amplifier stage with distortion detection, in particular a final amplifier stage with collector output and high dynamics.
In a low-frequency amplifier stage of the indicated type, shown by way of example and in simplified form in FIG. 1, the maximum output voltage is limited in theory by the supply voltage. Actually, in the best of hypotheses, the maximum excursion of the output voltage is equal to: EQU 2 V.sub.CC -V.sub.CEQ5,sat -V.sub.CEQ6,sat
When a low-frequency amplifier driving a loudspeaker with a for example musical signal reaches maximum excursion several times, it introduces a distortion due to clipping, so that the resulting sound becomes unpleasant.
A circuit can therefore be conceived which perceives this distortion condition, and sends this information, possibly filtered by means of an appropriate time constant such as to not introduce further distortions, to an attenuator circuit having the function of reducing the signal sent to the input of the amplifier so as to avoid, in stationary conditions, distortions of the output signal due to clipping higher than a given percentage.
For this purpose a circuit connected to the inputs of the amplifier stage has already been studied which can be schematically represented as an operational amplifier. In fact, as is known, in these types of amplifiers, during normal, linear operation, the voltage between the inputs is ideally nil, but becomes different from zero when the output is incapable of proportionally following the input, since the maximum possible output voltage has been reached (i.e. due to the occurrence of clipping). According to this known solution, therefore, the detector is connected to the inputs of the amplifier so as to detect any imbalances thereof indicative of the distortion. For this purpose, the known detector comprises at least two threshold amplifiers or comparators and an adder circuit.
Said known solution, though it indeed allows the detection of clipping, is however not free from disadvantages both due to the high circuital complexity and due to the interaction with the inputs which may be harmful in view, for example, of the decrease of the input impedance of the amplifier stage and of the increase of the input bias current.